noun

This is succeeded by plane-bedded sands dipping gently seaward, which are produced by the swash and backwash of the waves on the beach face.

Though I could no longer see the clam, I knew it had pushed its siphon to the surface for feeding, and it occurred to me that the hydroid, by creating an eddy in the swash, might actually help the clam obtain food.

1.1 archaic The motion or sound of water dashing or washing against something:the swash of the sea

Origin

Mid 16th century (in the sense 'make a noise like swords clashing or beating on shields'): imitative.